1988 244 DL N/A with about 300k miles.

History:

Driving today, it started stumbling. I pulled over, turned off the engine, and looked in the engine bay. Nothing stood out. I resumed driving. About 1 mile later, stumbling recurred. I got a tow home, car pointing up-hill. No start.

What I did:
Battery charged to 12.6 Volts. No start.
I felt the main fuel tank vibrate on turning key to "on" position; it did not sound unusual
Noid tester showed power to all fuel injectors
spark tester showed spark to all spark plugs
spark plug 1 sampled; dry and clean (just changed 100 miles ago with NGK copper)
2 Ohms resistance at fuses #4 and #6
No sound from inside fuel tank at gas fill area

Gas tank less than half full. Filled to 3/4 tank. Car started right up.

Tested wiring to in-tank/pre-pump engine idling. No current at black sheathed brown and gray wires (ammeter used). Backprobed at wire harness to test voltage. 12.76 volts between wire harness black wire and brown ground (which came from in in tank fuel pump). Only 7.1 Volts between the the black wire and gray wire that have the black sheath. I cleaned the ground and found only 1.2 Ohms of resistance between another bare metal part of the metal trunk 6 inches away and the wire.

Is the in-tank fuel pump bad or is there a problem with the wiring?

What do I do next?

How do I disconnect the black plastic wire harness plug that feeds the in-tank fuel pump?

Thanks